Generally speaking, automatic transmissions are essentially modulating converters that stepwise or steplessly independently change the present transmission ratio as a function of current or expected operating conditions, such as partial load, thrust, and ambient parameters such as temperature, air pressure, and humidity. Included in that group are modulating converters that are based on electric, pneumatic, hydrodynamic, or hydrostatic principles, or a combination of those principles.
A continuously variable transmission (CVT) normally includes a first shaft which can receive torque from a prime mover, a second shaft which is or can be parallel with the first shaft, an adjustable pulley or sheave on each of the two shafts, and an endless flexible element (such as a chain or a belt and hereinafter referred to as chain) trained over the two pulleys to transmit torque from the first shaft to the second shaft when the first shaft is driven by the prime mover. The two pulleys are adjustable and, to this end, each pulley comprises a first conical flange, which is affixed to the respective shaft, and a second conical flange which is rotatable with the first flange and is movable axially relative to the respective shaft toward and away from the respective first flange. Such adjustability of the pulleys enables the chain to move one of its looped portions radially inwardly toward one of the shafts while its other looped portion moves radially outwardly and away from the other shaft, or vice versa.
Continuously variable transmissions (CVTs) are often preferred over automatic transmissions, which employ a hydrokinetic torque converter in combination with a so-called bypass or lockup clutch. The reason is that the continuously variable transmission provides a greater comfort to the occupant or occupants of the motor vehicle because the shifts into different gear ratios invariably take place gradually without any appreciable shocks. Moreover, the utilization of a continuously variable transmission in the power train entails substantial savings in fuel requirements of the motor vehicle.
However, due to the physical limitations of continuously variable transmissions using the “cone-pulley” concept or a belt/chain driven system, conventional CVTs can only be used for low torque scenarios, such as small passenger vehicles and light duty machinery. Furthermore, the cones or belt/chains of a CVT must be kept tight in order to supply enough friction to be able to successfully transfer power and to prevent “slipping.” This energy that is used to keep the cones and belt/chains tight is effectively another form of wasting energy and thus reducing overall efficiency.
Moreover, in conventional CVTs, there is a “lag” time from when the gas pedal is depressed to when the car accelerates, which is also due to the physical limitations of the conventional CVT system of cone-pulley or belt/chains resulting in slower acceleration and insufficient torque. Also, conventional CVTs have the issues of noise, vibration and harshness. And since the structure of conventional CVTs is complicated, the reliability and durability thereof are decreased. Therefore, there remains a need for an improved shift system to overcome the problems stated above.